Semiconductor die are most commonly manufactured with their bond pads laterally located along the edge of either end of the rectangular die. To package this type of die, the bottom (non-circuit side) of the completed die is enjoined with a lead frame or package by gluing, stuck on with a type of double-sided tape, adhered with metal (eutectic), by pressure bonding with metal, or by various other means.
After the die is adhered to the lead frame or package, bond wires are attached to the bond pads and to the lead frame. These bond wires are a bridge between the package leads of the lead frame and the die, thereby allowing the storage and retrieval of information from the die by the electrical device into which the packaged die is installed. The bond wires, which are manufactured from gold or aluminum, are kept as short as possible as long bond wires decrease component performance due to increased signal propagation delay. Cost is also a factor, especially when gold bond wires are used.
After attachment of the bond wires, the die is packaged in either a plastic or ceramic encapsulation material. Encapsulating the die protects it from damaging contact with foreign bodies and prevents light, which can affect the charge stored on the cell capacitors, from reaching the cells on the surface of the die.
One problem associated with the lead frame of this assembly is that as the design of the die is changed to reduce its size, the lead frame must be made smaller in order to keep the bond wires short. This can create a waste of materials after a die shrink has been implemented. It also requires a new lead frame to be designed and manufactured.
Another type of die, which has its die pads running lengthwise down the center of the rectangular die, is becoming more common in the semiconductor industry. This type of die is described in U.S. Pat. No. 4,82,245. A die with centrally located die pads provides a packaged semiconductor chip which has improved mechanical, electrical, and thermal performance. Also, since a die shrink doesn't greatly affect the positions of the die pads, shrinking the die doesn't require a redesigned lead frame. A related feature is that a die with this type of design allows the ends of the lead frame to come into closer proximity to the die pads. Shorter bond wires can be used, thus improving performance of the packaged semiconductor. Shorter bond wires also reduce the cost of the package due to the reduction of the amount of gold used.
Die with centrally located bond pads are packaged in much the same way as die with laterally located pads. To position the bond wire connection points on the lead frame as close as possible to the bond pads, thereby reducing the length of the bond wires, the lead frame is enjoined to the top (the circuit side) of the die. To prevent shorting of the die by the lead frame, a layer of passivation coats the surface of the die. The die is then wirebonded to the lead frame, and the assembly is encapsulated.